In the 1950s and 60s the U.S. Air Force tested flying rockets and ramjets powered by nuclear reactors.
The Rover Kiwi-A engine undergoes testing. Photo: Los Alamos National Laboratory
The nuclear rocket engine idea has been revived a few times since then, mostly when engineers start to seriously strategize manned missions to Mars. It was also briefly revived as a method of getting the Strategic Defense Initiative ballistic missile shield into orbit.
Project Pluto was even more successful from a technical standpoint. Each ramjet test reactor achieved every one of its major goals, and a number of the tests were declared flawless.
Like the Rover, all the tests were conducted on the ground. Also like Project Rover, Pluto was shut down in favor of chemical propulsion. America had found a way to strike the Soviet Union from across the world without having to fly nuclear reactors over their own land and troops.
Both concepts and their accompanying research are mothballed, waiting for a mission to potentially revive them.